Tribological system of an internal combustion engine with a coating

ABSTRACT

The present invention relates to a tribological system (1; 2) comprising a first body (102; 105) and a second body (101; 107), which each form a component of an internal combustion engine (100), in particular a piston (102), a piston ring (105) or a cylinder (101, 107), and the surfaces (112, 108, 113) of which have a first and a second material area (11, 12) which come into contact with each other at least in some regions during operation and form a tribological contact, wherein the first and/or the second material area (11, 12) is formed as a layer on the basis of chromium oxide or aluminum chromium oxide. The invention also relates to an internal combustion engine (100) having such a system (1; 2).

TECHNICAL FIELD

The present invention relates in general and in particular to atribological system with a first body and a second body, which each forma component of an internal combustion engine, in particular a piston, apiston ring or a cylinder, and the surfaces of which have a first and asecond material area which are each formed as layers and come intocontact with each other at least in some regions during operation andform a tribological contact.

The disclosure relates to both the layer materials which are used forproducing the layers, and the methods, by means of which these layersare applied to the corresponding engine components. It also refers tothe correspondingly coated engine components.

The disclosure additionally relates in particular to the selection ofthe combination of the layer materials with which the tribologicalpartners are coated in the tribological system.

TECHNICAL BACKGROUND

The term ‘internal combustion engine’ shall here include all internalcombustion engines, in particular reciprocating piston engines, whichcan be operated with fossil fuels, such as gasoline, diesel or gas fuels(CNG, LNG).

Increasing the operating temperatures in internal combustion engineswith the aim of better utilization of the fuel and the use of additives,such as biofuels, place higher demands on the thermal and chemicalstability of surfaces in gasoline, diesel and gas combustion engines.These higher demands can only be met to a limited extent or at greatexpense by a selection of the compact materials (also referred to asbulk materials) for the engine components.

The terms ‘compact materials’ or ‘bulk materials’ refer to the materialsof the substrates to be coated in order to improve the surfaceproperties thereof. They include all materials that can be coated: e.g.steel, Al, Inconel, etc., i.e. materials and engineering materials, fromwhich the components of an internal combustion engine are produced.Among the steels, e.g. 42CrMo4 is a typical material, from which pistonsare made.

One method for the cost-effective improvement of the properties ofcompact materials is the surface treatment thereof, for example bynitriding, physical vapor deposition (PVD) or by spraying coatings ofpowders or wires using thermal spraying methods (HVOF or plasma). Theseprocesses can be used to produce materials that cannot be produced ascompact materials, or can only be produced with great effort, but whichcan drastically influence the wear, friction and corrosion of thecompact material surfaces, even if the modification only affects arelatively thin area of the surface.

Normally, the surfaces of the tribological partners in an internalcombustion engine consist of different materials or different compactmaterials, which are optimized for a certain service life in order toreduce the number of service cycles, i.e. the service life is adapted.In such cases, the material or the layer thickness is usually adjusted.

In addition to the desire to reduce wear, reducing the friction lossesis another goal in the development of internal combustion engines. Alsounder this aspect, the targeted modification of the surface plays animportant role in the tribological system. In addition to the mechanicalproperties of the surfaces and their adaptation to one another, thesurface materials must also assume important functions, such as thewettability with the oils and a certain control of the chemicalreactions of additives and surfaces, without becoming chemicallyunstable themselves. All these facts show that the optimization of thetribological system of an internal combustion engine is extremelycomplex and requires flexibility in the selection and coordination ofthe surfaces. Thin layers offer here a higher degree of flexibility thanis the case with compact materials.

EP 1 022 351 B1 describes the coating of the inside of a cylinder, thecylinder running surface, wherein the layer is applied by plasmaspraying. The mostly ferrous layer also contains FeO and Fe2O3proportions, wherein the proportion of bound oxygen is between 1 and 4wt %- and the Fe2O3 proportion is below 0.2 wt %. The admixture of oxideceramic powders with a proportion of 5 to 50 wt % to the process gas isrecommended as a particular advantage in order to achieve even bettercoefficients of friction. TiO2, Al2O3-TiO2 and Al2O3-ZrO2 are indicatedas oxide ceramic powders.

EP 1 340 834 A2 claims a cylinder running surface layer forreciprocating piston engines, wherein this layer has been applied by aplasma spraying method. The layer produced in this way has a contentbetween 0.5 and 8 wt % of bound oxygen and includes embedded FeO andFe2O3 crystals. In addition, the layer has a porosity degree between 0.5and 10% and is honed to a certain roughness. The pores in the layer formmicrochambers which serve as a reservoir for lubricant and which promotea uniform oil distribution in the tribological system.

WO 2015/128178 A1 describes the surface treatment of a piston ring,which consists of a combination of surface nitriding and PVD coating.The running surface of the piston ring to the cylinder running surfaceis coated with a CrN layer, preferably deposited by ion plating. Allother surfaces are subjected to a nitriding process. The deposition ofthe PVD layer on the non-nitrided piston ring surface is primarilyintended to prevent cracking in the coating.

US 2014/0260 959 A1 claims a coated piston ring made of iron-basedmaterial, on the surface of which a wear-resistant layer is depositedwhich contains Al5Fe2 and which has a high hardness. This document alsoclaims a chemical composition of this layer consisting of 52 to 55 wt %Al and 45 to 48 wt % Fe.

U.S. Pat. No. 7,406,940 B2 describes a tribological system consisting ofa piston and a cylinder running surface, wherein the piston also hasrecesses for piston rings which are in tribological contact with thecylinder liner. In addition, the piston skirt is designed in such a waythat the surface thereof is provided with a trench structure to improvethe lubricating film. The piston ring and piston skirt are also coatedwith a DLC layer to improve the sliding properties, above all inconjunction with and adapted to a variety of lubricants.

The examples show that surfaces are coated to reduce the wear thereofand reduce friction losses. A variety of coating materials can be usedto achieve this goal. Different processes are employed to apply thesematerials to the substrate surfaces. Furthermore, the state of the artmakes it clear that very different substrate materials have to be coatedin order to achieve advantageous surface adjustments of the partners inthe tribological contact under the respective conditions.

In some cases, the geometry of the component to be coated also dictatesthe coating process. This is the case, for example, when cylinder boreshave to be coated. For such a coating of the inner part of thiscomponent, a spraying method is far more suitable than a PVD method.Even in cases where thicker layers of above 100 μm or even above 500 μmhave to be deposited, such methods are far more effective than a PVDcoating. The latter in rum has economic advantages for thinner layers inthe range up to 10 μm or up to 30 μm if the substrates can be coated ina batch process.

Testing the layer pairings of a special tribological system with regardto wear and friction losses could, of course, best be carried out on thereal internal combustion engine. However, these tests are too costly tocarry them out for all possible material combinations. In addition,there would also be the risk that unknown material combinations coulddamage the internal combustion engine, which could result in thedestruction of the entire test stand.

The above shows both the variety of possibilities and how difficult itis to make a selection of layers and layer pairings.

The object is therefore to provide an improved tribological system, inparticular for internal combustion engines, in which two bodies eachform components of an internal combustion engine and two material areasare formed on their surfaces, which touch each other at least in certainareas during operation and form a tribological contact. In particular,there is a need to improve tribological systems consisting of pistonring and cylinder or of cylinder and piston. The object is here also inparticular to provide further materials or material combinations fromwhich the material areas designed as layers can be produced.

SUMMARY

According to a first aspect, the present disclosure provides atribological system having a first and a second body, which each form acomponent of an internal combustion engine, in particular a piston, apiston ring or a cylinder, and the surfaces of which have a first and asecond material area which come into contact with each other at least insome regions during operation and form a tribological contact, whereinthe first and/or the second material area are/is formed as a layer onthe basis of chromium oxide or aluminum chromium oxide.

Further aspects and features follow from the dependent claims, theattached drawing and the following description of embodiments.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments are now described by way of example and with reference tothe attached drawing, wherein:

FIG. 1 shows a schematic sectional diagram with components from aninternal combustion engine according to the invention, which includestribological systems according to the invention;

FIG. 1A shows an enlarged section A of FIG. 1, which illustrates a firsttribological system according to the invention, said system being formedof a cylinder and a piston ring;

FIG. 1B shows an enlarged section B of FIG. 1, which illustrates asecond tribological system according to the invention, said system beingformed of a cylinder and a piston;

FIG. 1C shows a detail C of FIG. 1, which illustrates a cylinder bottomwith a coating;

FIG. 2 shows a schematic diagram of an SRV test setup for a dry test(left-hand illustration) and a lubricated test (right-hand illustration)for a tribological test for determining a coating suitable for atribological system according to the invention;

FIG. 3 shows the time course of the coefficients of friction for areference material A1 and an Al—CrO material D2 with a counter-body froma steel material;

FIG. 4 shows the observed and measured wear of the materials from FIG.3;

FIG. 5 shows the wear of the counter-body in the SRV test with thelayers illustrated in FIGS. 3 and 4;

FIG. 6 shows the time course of the coefficients of friction of thematerials A1 and D2 illustrated in FIGS. 3-5 with a counter-body made ofaluminum oxide;

FIG. 7 shows the observed and measured wear of layers A1 and D2 with thecounter-body according to FIG. 6;

FIG. 8 shows the wear of the counter-body after the SRV rest under dryconditions in contact with the coatings A1 and D2;

FIG. 9 shows the time course of the coefficient of friction of A1 and D2with a steel ball as a counter-body under lubricated test conditionswith a first lubricant;

FIG. 10 shows the wear of the layers A1 and D2 after the SRV test underlubricated conditions according to FIG. 9;

FIG. 11 shows the wear of the steel counter-body after the SRV testillustrated in FIGS. 9 and 10;

FIG. 12 shows the time course of the coefficient of friction of A1 andD2 with an aluminum oxide ball as a counter-body under lubricated testconditions with a first lubricant;

FIG. 13 shows the wear of the layers A1 and D2 after the SRV test underlubricated conditions according to FIGS. 11 and 12;

FIG. 14 shows the wear of the counter-body after the SRV test underlubricated conditions according to FIGS. 11-13;

FIGS. 15A to 15C show the time course of the coefficients of frictionfor the materials A1, D1 and D2 for a lubricated SRV test with analuminum oxide counter-body at room temperature (FIG. 15A), at 100degrees Celsius (FIG. 15B) and at 160 degrees Celsius (FIG. 15C); and

FIG. 16 shows the comparison of the layer wear and counter-body wear forthe lubricated SRV test illustrated in FIGS. 15A and 15C for the layersystems A1 and D2.

DESCRIPTION OF EMBODIMENTS

General explanations on the embodiments are first made, followed by adetailed description of the embodiment with reference to the drawings.

An embodiment, in which the tribological system comprises a first bodyand a second body, which each form a component of an internal combustionengine, in particular a piston, a piston ring or a cylinder, and thesurfaces of which comprise a first and a second material area which comeinto contact with each other at least in some regions during operationand form a tribological contact, wherein the first and/or the secondmaterial area is formed as a layer on the basis of chromium oxide oraluminum chromium oxide, has properties which are in particularfavorable for the critical tribological systems that are near thecombustion chamber in an internal combustion engine.

Layers on the basis of chromium oxide or aluminum chromium oxide can beapplied in different processes to different materials, including compactmaterials that are typical for internal combustion engines.

Steel (e.g. quality 42CrMo4) is a typical material from which pistonsare manufactured. Further typical materials for the pistons and theirrunning surfaces at the piston connector (also piston skirt) are e.g.peritectic aluminum-silicon alloys with 11-13% Si and minor additions ofCu, Mg and Ni. There are also light metal composite materials, in whichreinforcing elements, for example made of ceramics, carbon fibers andporous metallic materials, are specifically arranged in particularlyhighly stressed piston regions.

The cylinders are usually part of the crankcase and can be manufacturedin monometallic design, e.g. from cast iron alloys or aluminum-siliconalloys. The running surfaces of such cylinders are formed directly onthe material for casting.

In the so-called application technique, cylinder sleeves are insertedinto the crankcase. In this case, the sleeves are made of GCI materials,aluminum materials or also aluminum-silicon alloys and are inserted orpressed, shrunk or cast into the crankcase in appropriately machinedreceptacles.

In the so-called composite technique, cylindrical shaped bodies made ofa composite of suitable metallic and ceramic materials are inserted intothe casting mold and infiltrated under high pressure by the aluminumalloy melt forming the crankcase.

The cylinder running surfaces of all cylinders are designed astribological running partners and sealing surfaces for pistons andpiston rings by fine boring or turning and subsequent honing.

A variety of designs and materials exist for the piston rings, whichserve as a metallic seal and seal the combustion chamber against thecrankcase. Steel or grey cast iron materials are widely used. In thiscase, it is also common practice to reinforce the ring running surfaces,which form a tribological system with the cylinder slide way, withwear-resistant protective layers. In addition to the already mentionedcoatings for the ring running surfaces, chromium-plating, chrome-ceramiclayers, chrome-diamond layers or molybdenum-based coatings are alsoprovided. In nitriding or nitrocarburizing processes, nitrogen and insome cases also carbon can be incorporated into the surface of thepiston ring by diffusion.

There are designs where the first and second material area, which aremade as a coating, are each arranged on a surface of the components thatforms a running surface. For the components, i.e. piston, piston ring orcylinder, these are the piston shaft slide ways on the piston, the ringrunning surfaces on the piston rings and the cylinder slide ways on theinside of the cylinders. These surfaces forming the respective runningsurfaces each form a tribological system according to the invention inpairs and can be designed as a layer on the basis of chromium oxide oraluminum chromium oxide.

There are designs where the first material area on a first body formedas a piston ring is a layer on the basis of aluminum chromium oxide witha chemical composition of (Al1-xCr-y)2O3, wherein: 0.1≤x≤1 and y≤0.5;0.5≤x≤1 and y≤0.5 or x=0.7 and y=0.3. Such a coating shows positivetribological properties in both the dry state and the lubricated statefor different friction partners (second material area on the cylinderrunning surface).

In other designs, the second material area on a second body formed as acylinder is a mixed alumina-chromium oxide layer having a composition of10 wt % to 100 wt % Cr2O3 and correspondingly 90 wt % to 0 wt % Al2O3.Such a layer is to be applied to a cylinder running surface by suitablecoating methods (e.g. by means of thermal spraying) and is a goodtribological partner for another layer on the basis of aluminum chromiumoxide (formed by the first material area) with the above indicatedcomposition of (Al1-xCr-y)2O3.

There are designs where the tribological properties can be furtherimproved by favorably influencing the surface roughness. Here, thefollowing values for the surface roughness Ra in μm apply to the firstmaterial areas: 0.1≤Ra≤0.5; 0.15≤Ra≤0.4; Ra=0.15 or Ra=0.4 and thefollowing values apply to the second material areas: 0.1≤Ra≤0.5;0.15≤Ra≤0.45; Ra=0.15 or Ra=0.45.

There are designs where the material areas are formed by means of athermal spraying method. A spray material (this can be e.g. a powdermixture with different components, such as Al2O3 and Cr2O3) isintroduced into a concentrated, high-energy heat source, where it isfused or partially melted and thrown at high speed onto the surface ofthe substrate to be coated in the form of spray particles. This processis particularly suitable for coating larger surfaces such as thecylinder running surface or also the piston shaft slide way. Commonmethods are flame spraying, high-speed flame spraying (HVOF), so-calledcold gas spraying, arc spraying and plasma spraying. The resultingcomposition of the layer corresponds approximately to the composition ofthe starting material (the powder mixture).

In thermal spraying, layer thicknesses are between 50 mm and 400 mm, inparticular also between 150 μm and 800 μm.

There are also designs where one of the materials is formed by means ofa PVD method (physical vapor deposition) and in particular by means ofcathodic spark evaporation. In the known PVD methods, the deposition ofions, atoms or molecules from the gas phase or plasma producestribological protective layers on the surface of the substrate (basematerial of the component to be coated). For this purpose, the requiredstarting materials (e.g. metals, ceramics, etc.) are thermallyevaporated or atomized and condensed again on the components.

Both the so-called cathodic spark evaporation and the so-called cathodeatomization (or MSIP—magnetron sputter ion plating) have beenestablished processes for quite some time, which are used for coatingtools and components and by means of which the most different layers canbe deposited.

For example, there are designs where the material areas formed by a PVDmethod have a layer thickness of between 10 and 30 μm.

There are also designs where one material area is formed as a layer onthe basis of Mo, MoN, MoCuN, DLC or ta-C and one material area is formedas a layer on the basis of chromium oxide or aluminum chromium oxide.

Mo, MoN, MoCuN are resistant layers which improve the tribologicalproperties.

DLC (diamond like carbon) layers generally comprise so-called thin-filmsystems, which essentially consist of carbon and are applied using PVDor CVD (chemical vapor deposition) methods. These layers are graphitic,diamond-like or amorphous carbon or carbon hydrogen layers as well asplasma polymer layers.

Layers on the basis of ta-C are also so-called DLC layers anddistinguish themselves by a certain ratio between graphitically bondedcarbon and diamond-like bonded carbon, in which the diamond-like bondedportion predominates. Such layers on the basis of ta-C also offerexcellent tribological properties, in particular also in tribologicalcombination with layers on the basis of chromium oxide or aluminumchromium oxide.

There are designs where at least one of the layers has a universalhardness of at least 80 GPa. The hardness of a layer is often anindication of the wear resistance.

The invention concerns in particular an internal combustion engine withan tribological system according to the invention. The above explainedtribological systems are particularly advantageous in the thermallyhighly stressed components with sliding friction loads in thecylinder/piston area. These systems include the tribological systembetween piston ring 8/9 (rings) and cylinder slide way as well as thetribological system which is formed of the piston shaft slide way (alsopiston skirt slide way) and the cylinder slide way.

There are designs where the piston is provided with at least one furthersurface area which is formed as a layer on the basis of chromium oxideor aluminum chromium oxide. These areas include in particular the pistonring groove areas, the piston crown or also the piston shaft. Here,thermally resistant and robust hard layers are helpful to transfer thehigh stresses in these areas into the base material.

Returning to FIG. 1, this figure illustrates, in a schematiclongitudinal sectional diagram, components of an internal combustionengine 100 with a cylinder 101, in which a piston 102 is arranged, whichoscillates in operation and which is provided with three piston rings103 that are arranged in piston ring grooves 104. The cylinder wall 105has a cooling channel 106, and the actual cylinder bore is formed hereby an inserted (optional) cylinder liner 107, on the inner surface 108of which the cylinder running surface is formed.

The piston 102 has at its upper end the piston crown 109 on which acylinder-shaped piston ring groove area 110 borders that merges into thepiston connector 111.

The cylinder 101 or cylinder sleeve 107 (if present), on the one hand,and the piston rings 103 or the piston connector 111, on the other hand,form here the tribological systems 1 and 2.

FIG. 1A shows a tribological system 1, in which the piston ring 103forms a first body, on the surface of which, at least in the area of thering running surface 112, a first material area 11 is formed as a layeron the basis of aluminum chromium oxide, e.g. as an (Al1-0.7 Cr-0.3)2O3layer. As regards other compositions according to the invention, thefollowing may apply (Al1-xCr-y)2O3, wherein: 0.1≤x≤1 and y≤0.5; 0.5≤x≤1and y≤0.5. The layer thickness is between 10 and 30 μm and has ahardness of at least 18 GPa. The layer has a surface roughness (Ra inμm) of Ra=0.15 or Ra=0.4 and is in other embodiments within a rangewherein: 0.1<Ra<0.5. Other suitable coatings for the piston rings 103are designated in table 1b as C2 and D2.

A second material area 12 is formed as a layer on the cylinder slide way108, said layer being formed from an alumina and chromium oxide mixtureand having a weight content of 62 wt % Al2O3 and a weight content of 38wt % Cr2O3. The surface roughness is Ra=0.45 μm (E1 in Table 1a).Alternatively, the first material area is formed as a chromium oxide(Cr2O3) layer which has a surface roughness of Ra=0.15 μm (D1 in Table1a).

Alternatively, the second material area 12 is formed from an aluminumchromium oxide layer in which the chromium oxide content constitutes awt % content of 10 to 100 and the aluminum oxide content forms acorresponding complementary wt % content of 90 to 0. Other suitablecoatings for the second material area can also be the layer systemsdesignated A1, B1 and C1 in Table 1a.

The piston ring or rings 103 (first body) and the cylinder 105 or thecylinder sleeve 107 (second body) thus form a tribological system withthe contacting surfaces 112 and 108 and the first material area or areas11 and the second material area 12, respectively, in which thecontacting surfaces glide against each other during operation.

There are operating conditions in which there is a largely dry friction,namely at the upper and lower dead center of the piston, at which thepiston speed is low or even zero. The lubricating film between the twosurfaces formed by the engine oil then largely breaks off.

Wet or lubricated friction occurs during the oscillating movement of thepiston between the reversal points. The engine oil then forms aneffective lubricating film between the material areas 11 and 12, whichgreatly reduces friction and wear.

Mixed friction conditions can also occur in which lubricants are notpresent as a complete film between the material areas 11 and 12 but areavailable in depressions in the surface. This lubrication effect can bestronger with higher surface roughness (e.g. on honed surfaces) thanwith smoother surfaces with lower surface roughness (e.g. polished orbrushed surfaces), which have a lower lubricant absorption capacity.

A second tribological system 2 is shown in FIG. 1B. The detailedillustration 1B shows the piston connector 111 on the surface 113 ofwhich a first material area 11 is formed, which is provided with acoating, which are designated in Table 1a as A1, B1, C1, D1 or E1.Alternatively, the surface 113 can also have a different coating, whichis described above in connection with the coating of the cylinder slideway 108. Here too, similar to the first tribological system 1, dry,lubricated or mixed friction occurs during operation, which depends onthe state of movement of the piston 102 in the cylinder 105.

Further optional coatings can also be provided in other surface areas ofthe piston; on the one hand, at the piston crown (see FIG. 1C), whichcan be provided with a layer 114 to improve the thermal and pressureresistance. Another layer 115 can also be provided in the piston ringgroove area 110, in particular in the area of the piston ring grooves104. The layers A1, B1, C1, D1, E1, listed in Table 1a, and also layerson the basis of molybdenum, molybdenum nitrogen, molybdenum coppernitrogen, DLC or ta-C can be used here.

Furthermore, it also applies that, for example, if the piston rings 103are provided with a layer on the basis of chromium oxide or aluminumchromium oxide (in the first material area 11), the layer cooperatingtherewith (second material area 12) can be formed as a layer on thebasis of Mo, MoN, MoCuN, DLC or ta-C on the cylinder slide way 108.

Conversely, in the case where the piston ring or rings 103 are providedwith such a layer, the cylinder slide way 108 is then provided with alayer on the basis of chromium oxide or aluminum chromium oxide, inparticular with a layer, the properties of which are described in derailabove or below.

The same applies to the tribological system formed by the cylinder slideway 111 with the piston shaft slide way 113.

In the following, FIGS. 2 to 16 are used to describe a test with theassociated test setup and some test results, which have led, among otherthings, to the above described embodiments according to the invention.

An important test of this kind is the vibration-friction-wear (SRV) test(DIN 51834), with which the wear and friction behavior of materials canbe characterized. With these tests it is possible to simulate manytribological situations of the later application, so that a usefulpreselection of the coatings of body and counter-body can be made. It isthus possible to use in the real combustion engine a mostly alreadysomewhat optimized solution when selecting the coatings, so that thenonly fine adjustments are necessary. This is achieved by adjusting theparameters of the SRV test, such as contact pressure or temperature, insuch a way that the result of the test reflects the results of enginetests already performed.

Such an SRV test was used for the investigations according to theinvention, and the parameters for the test were selected in such a waythat deficient lubrication in the tribological contact was simulated.The parameters were used both in the case of deficient lubrication andin the case where the test was carried out “dry”, i.e. withoutlubricant. Experience has shown that such a test provides informationabout two basic wear behaviors. The dry test investigates above all theseizure behavior of the partners in tribological contact and therelative wear of the tribo-partners becomes clear. The lubricated testsunder this high contact pressure simulate insufficient lubrication.These test conditions provide information on the wear of thetribo-partners and also provide coefficients of friction that allowrelative comparisons of the material combinations.

Various layer materials were investigated in the experiments. Theselection of the layers was carried out with a view to furtherdeveloping and improving the tribological contact between piston ringand cylinder slide way of an internal combustion engine, i.e. the layerswere produced in a first group 1 using a thermal spraying method and ina second group 2 using a PVD method, preferably by sputtering and morepreferably by reactive cathodic spark evaporation.

The two coating methods were used to coat the test specimens, namely thesample specimens of group 1 were provided with layers which, asexperience has shown, are well suited for application to cylinder slideways or which were to be examined for this application.

Group 2 specimens were coated with materials which, as experience hasshown, are suitable for piston ring coatings or the properties of whichshould be clarified for such an application. The layers were depositedon planar test specimens and partially after-treated. The test wascarried out with both steel counter-bodies (100Cr6) and aluminum oxide.

FIG. 2 shows the schematic setup of the apparatus which was used forcarrying out the SRV tests and describes the terms used in the text. Thetests were carried out for a far greater number of layer materials, butnot all of them are discussed here.

Two important technical goals aimed at in the development in the fieldof internal combustion engines are to reduce wear in order to prolongthe service lives of the components and the service interval times aswell as to reduce friction losses in order to improve the efficiency.

An important aspect also concerns the temperature stability of thematerials used in the internal combustion engine. The trend here is toimprove the temperature stability so that the combustion process cantake place at higher temperatures in order to thereby also increase theefficiency of the engine. The search for temperature-stable layermaterials that protect the compact material surface at these highertemperatures and have good tribological properties at both ambienttemperature and high temperatures was another important reason for theseinvestigations.

FIG. 2 describes the principle of the SRV test under dry (on the left)and lubricated (on the right) conditions. The tribological system ortribo-system consists of two tribo-partners—the body (K) and thecounter-body (GK). While the K is coated with a layer, the experimentsused spherical polished GK made of steel (100Cr6) and aluminum oxide. Kis firmly clamped in a base and can be heated. GK is moved in anoscillating manner with horizontal oscillations above K by the forcesindicated by the two arrows over the coated surface of K while a load Lis simultaneously applied. In the lubricated test (on the right), alubricant S is added to this oscillation.

Description of the Experiments

Tables 1a and b show the layer materials that have been tested. Some ofthese materials have already been technically introduced or have alreadyqualified for engine tests (A1, B1, C1).

In addition, further materials were tested, the tribological propertiesof which were unknown but which promised improved temperature stability(D1, E1, C2, D2). These materials are mainly of oxidic origin and someof them belong to the content of the invention. Tables 1a and b listonly some of the materials tested and also indicate the surfaceroughness which these layers had. It depends, of course, on thedifferent after-treatments.

These investigations only used common methods for theseafter-treatments, such as honing, grinding or brushing, so that atechnical implementation of the invention is ensured not only for thecoating but also for the after-treatment. The table groups the layermaterials according to the coating methods, i.e. the layers for whichthe application to the cylinder running surface is intended are given inTable 1a, those intended for the coating of the piston rings are givenin Table 1b. This separation was made because the different coatingmethods are suitable for the coating of ring and liner to differentdegrees, but it should not be regarded as a restriction.

Furthermore, it is not mandatory that layer materials with the samechemical composition, which were produced in the different coatingmethods, also have the same properties in the tribological system. Thiscan be due, for example, to different layer thicknesses or themechanical properties of the layers with regard to their porosity orresidual stress. Thermal spraying methods were used for the layers onthe cylinder running surfaces, and coatings with thicknesses between 150μm and 800 μm were produced. For the coating of the piston ringmaterials a PVD method was used, which was the reactive cathodic sparkevaporation for the layers selected in this case, although other coatingmethods, such as thermal evaporation or sputtering, can also be used forthis purpose.

Flat samples were coated. All samples produced by thermal spraying wereafter-treated in such a way that they had a surface roughnesscorresponding to the standard honing which is used in production for theafter-treatment of the thermally sprayed cylinder slide ways.

The layers produced by means of spark evaporation were either notafter-treated or were after-treated using a standard brushing method.The assignment to the manufacturing process, a selection and moredetailed description of the layer materials as well as the type ofafter-treatment and the resulting surface roughness are also given inTables 1a and b.

Polished steel (100Cr6) and aluminum oxide balls were used ascounter-bodies in the SRV tests. The tests were carried out both dry andlubricated with two different oils. The test was carried out at ambienttemperature and for temperatures up to 160° C., close to the stabilitylimit of the lubricant used, in order to also investigate the stabilityof the layers at higher temperatures.

The tests yielded the following results:

1. The curve of the coefficient of friction as a function of time

2. The wear of the coating under the various conditions

3. The wear of the counter-body under the various conditions

The results are given in tabular form. For the coefficient of friction,the value resulting at the end of each test is incorporated into thetable. The evaluation of the wear of coating and counter-body is basedon the optical assessment of light microscopic wear images and thequantitative evaluation of optically scanned surface profiles. For abetter understanding of the values given in tabular form, an example isgiven for all types of results and the conditions examined in the SRVtest.

FIG. 3 shows the time course of the coefficient of friction for thelayer of the after-treated low-alloy steel (A1) and the polished Al—Cr—Olayer (D2) with the polished steel ball as a counter-body. The SRV testwas carried out dry, i.e. without lubrication. It can be seen that thecoefficient of friction at the end of the measurement time is 0.86 forA1 and 0.76 for D2. The noise of curve D2 is higher with the oxide layerthan with the low-alloy iron.

Under these test conditions, the tribological system has values for thecoefficient of friction that indicate to a person skilled in the art a“seizure” in the tribological contact. Such values must be avoided forthe applications discussed here.

FIG. 4 shows the wear of the layer. For this purpose, light microscopicimages of the layer surface were taken after the test (upper row ofimages). A clear change in the layer surface can be seen in the area inwhich the oscillating steel ball has moved and which is about 1 mm long.EDX measurements can be used to detect the material of the counter-body(100Cr6) on both A1 and D2, i.e. material was transferred from thecounter-body to the layer (seizure), indicating wear of thecounter-body.

To characterize the layer wear in more derail, the surface wasmechanically scanned perpendicular to the oscillating direction ofmovement, as indicated by the white line in the illustration. A confocalwhite light microscope of type “μ-Surf” of the manufacturer Nanofocuswas used as the device for this measurement. The surface profile for A1obtained in the scanning shows an increase in the surface roughness withpartial material application and partial layer wear of A1. Less wear canbe observed in the surface profile of D2: Application at the edges ofthe oscillating movement and almost no abrasion in the middle, exceptfor a scratch. On the scale of wear (1—no wear, 5—strong wear), thiswear would be evaluated with about 2 for A1 and 1-2 for D2.

The size of the coefficient of friction can also be explained to aperson skilled in the tribological field. The values around 0.8 aretypical for a dry friction of steel on steel, which in this case isrealized by transferring the material to the layer. Further wear valuesare given in the table 2. In the values for B1 and C2, a minus sign wasplaced after the wear value, which is supposed to indicate that a verystrong material transfer of the counter-body took place in these twolayers, i.e. there was no wear but a layer build-up with thecounter-body material. The values in the table indicate that, with theexception of C1, the layers are highly wear resistant to steel. However,they indicate that dry conditions must be avoided for the discussedlayers if, in the tribological system, a steel wear-resistant layer iscombined with steel as the counter-body. Under these conditions, evenimproved surface roughness can apparently not prevent the counter-bodymaterial against seizure.

In addition to the wear of the coated body, in a tribological system thewear of the counter body is, of course, also important when it comes tooptimizing the wear behavior of the tribo-system. FIG. 5 thereforeexamines the wear of the counter-body, in this case, the polished 100Cr6steel ball by taking light microscopic images of the ball area that wasin contact with the layer. The diameter of the so-called wear cap is ameasure of wear and the wear volume can still be calculated therefrom.For the tabular evaluation, a scale from 1 to 5 is used here again,wherein 1 again means no wear and 5 means very high wear. Regarding thephotos in FIG. 5 A1 is assessed with 4 and D2 with 3.

What also stands out is the lower noise over time in the coefficient offriction for D2. The examination of the layer wear using lightmicroscopic images and surface profiles in the wear area gives differentresults for the two layers (FIG. 7). A clear layer wear is observed inconnection with A1, which is estimated to be 4. Amazingly enough, nolayer wear can be found with D2, there is only a kind of smoothing ofthe contact area. Based on this, the layer wear is evaluated with 1.

The wear of the aluminum oxide counter-body is shown in FIG. 8. The wearof the counter-body is clearly smaller for both layer systems than isthe case with the 100Cr6 counter-body. For A1 (top left), thecounter-body shows a contact surface which has many scratches.Subsequent measurements of the rounding of the counter-body (bottomleft) confirm a roughening of the surface of the aluminum oxidecounter-body, although otherwise no significant change in the ballradius could be observed. It can be assumed that this material pairinggenerates breakouts which lead to such a rougher surface. The backgroundto these outbreaks has not yet been unequivocally clarified. No wear ofthe counter-body is observed for D2 (right-hand column). This is anamazing result for a tribological system without lubrication, which isoperated under high contact pressure. The circle area in the photos withmodified surface quality for A1 and D2 indicates the area of deformationwhen the test proceeds under the selected conditions and characterizesonly the deformation in the region of the contact area of body andcounter-body, which is typical for the respective material pairing. Thisdeformation is therefore not necessarily a wear cap as was the case forthe 100Cr6 ball. The proof of this is once again delivered by thescanning of the surface with the nanofocus, which gives the originalradius and shows no flattening of the ball. The outstanding results forthe dry SRV test are therefore provided by the bodies and counter-bodiesin which layer pairings of layers on the basis of chromium oxide andaluminum chromium oxide are available.

The results of the SRV test with lubricant are presented below. Thetests were carried out with various lubricants which led to differentcoefficients of friction. In terms of wear, however, these differencesare less or not differentiable. Usually only a different coloring of thecontact area is visible. Examinations showed that this is mainly due tothe different additives in the oil. Since the treatment of the additivesis not the subject of this invention, these differences are not dealtwith in more detail.

FIG. 9 shows the coefficient of friction over time for A1 (1) and D2(2), which was measured with diesel oil as lubricant (here referred toas oil1) and the steel ball as counter-body. A1 has a coefficient offriction of 0.20 in run-in operation, while that of D2 is 0.18. Therelative differences in the coefficients of friction in a comparisonwith the dry test have become smaller, i.e. the material influence underlubricated conditions is less pronounced. The surface quality of thecoating also has a greater influence, i.e. that the hole or porestructure of a surface—as present in A1 as a result of the thermalspraying process—is more suitable to keep the lubricant in the surfaceand thus makes the tribological system safer, especially in the case ofinsufficient lubrication. In outlines, this can also be seen whencomparing C2 and D2 because the unpolished layer C2 has a lowercoefficient of friction than the polished one. Table 3 shows allcoefficients of friction for the lubricated SRV test together with thesteel as the counter-body.

The layer wear obtained under these test conditions is shown in FIG. 10.A1 shows low layer wear with a value of 2 and otherwise only a smoothingeffect in the contact area. No layer wear (value 1) can be measured forD2. Here, too, smoothing could take place although it is not clearlydemonstrated in the measured surface profile. The other values for layerwear are again given in table 3. Low wear can only be measured forlayers A1, B1 and C1 while all layers on the basis of chromium oxide andaluminum chromium oxide show no wear.

The wear of the steel counter-body with the lubricant oil1 is shown inFIG. 11. Surface discolorations can be seen for both configurations. Thesurface profile of the 100Cr6 ball in the case of A1 shows little wear(value 1-2), i.e. the surface discoloration is caused by an elasticdeformation of the contact surface alone and the discoloration is causedby a thin film which mainly resulted from the additives in the oil orthe decomposition products thereof. For D2, the size of the contactsurface discoloration is similar to that of A1, but the surface profileof the 100Cr6 ball shows low wear with the value 2. The steel body issubject to more pronounced wear by the layers E1 and C2. All layers onthe basis of chromium oxide and aluminum chromium oxide show a very highmicrohardness of over 20 GPa (measured according to the ISO 14577standard), and it is important that the surface quality is chosenaccordingly if these layers are to be used with steel as a counter-body.

The result of the SRV test with the aluminum oxide counter-body is ofgreat interest because no wear could already be determined under dryconditions on the two tribo-partners for D2. FIG. 10 again shows thetime course of the coefficient of friction, for which a value of 0.19results for A1 and of 0.18 for D2 at the end of the test. Here, too, therelative influence of the coating material on the coefficient offriction is lower than in the dry test. Obviously the formation of asurface, which results from the interaction of material surface, oil andadditives and which manifests itself in the discoloration, plays animportant role. B1 has the highest coefficient of friction. Otherwise, acoefficient of friction of 0.18 was measured for all other materials. Inthese tests, the coefficient of friction is surprisingly lower than inthe case of A1 and C1 layers already introduced against steel.

FIG. 13 shows the layer wear, which is very low for A1 (value 2) andrather only indicates a smoothing effect. This is a good behavior,especially against the background that the layer contains many pores.This in turn indicates that such a surface geometry contributes tobetter tribological properties, i.e. that lubricants are retained in thelayer recesses and reduce friction losses. No layer wear can be measuredfor D2 (value 1). The layer is stable, although a certain smoothingeffect is also suspected here. High wear show B1 (value 5) and also C1(value 4). In particular layer B1 shows how important it is tocoordinate body and counter-body, because B1 provides the lowestcoefficient of friction against steel and the highest against aluminumoxide.

FIG. 14 characterizes the wear of the aluminum oxide counter-body.Neither A1 nor D2 show any wear of the ball (both values 1). Theroughness in the surface area, which is measured in the surface profile,is characteristic for the given porosity of the material. The scratchesare caused by small eruptions of the material, which during the testoccur between the surfaces of the layer and the counter-body and, incontrast to real engine tests, are not transported away with the oilduring our test setup.

Tables 2 and 3 summarize the results for the layer materials tested withrespect to the two counter-body materials for the different testconditions. The qualitative estimates were derived from themeasurements, as exemplified in the previous text. On the basis of theseinvestigations, the results of the SRV test so far can be summarized asfollows:

SRV: dry with 100Cr6 as counter-body

-   -   For all tests there is considerable wear of the 100Cr6 steel        counter-body    -   The tests stabilize at coefficients of friction in the range        around 0.8. This is due to the fact that material of the        counter-body is lubricated (transferred) to the respective        layer, thus creating a steel-steel contact. Accordingly, a clear        and high wear of the counter-body is observed for all layers        examined here.        SRV: dry with aluminum oxide as counter-body    -   The layers A1, B1 and C1 show layer wear, although this wear is        lower for the layers selected here compared to other common        materials that were not included in the table.    -   The oxide layers investigated are divided into two groups. The        group with high surface roughness (E1) has a coefficient of        friction around 0.8 and shows a strong noise in the time course        of the coefficient of friction. The second group (D1, C2, D2)        has coefficients of friction around 0.6 and thus stands out        clearly from the other layers. The layers of this group have a        surface roughness of Ra below 0.4 μm.    -   Apart from smoothing effects, all oxide layers based on Cr and        Al—Cr (D1, C2, D2) show no layer wear.        SRV: lubricated with 100Cr6 as counter-body    -   The coefficients of friction for all the layers examined show        values below 0.2, with B1 and D1 being the lowest.    -   The layers (A1, B1, C1) which are not based on chromium oxide        and aluminum chromium oxide show low wear of value 2.    -   No layer wear can be detected on the layers (D1, C2, D2) on the        basis of chromium oxide and aluminum chromium oxide.    -   With the exception of E1 and C2, i.e. the hard oxidic layers        with greater surface roughness, all other layer materials have        small counter-body wear values of 1-2 or 2. The surface        roughness of the layer under lubricated conditions and with        100Cr6 as counter-body obviously plays an important role, and if        such material pairings are to be realized, the running-in        behavior must be optimized and the layer roughness Ra must be        less than 0.2 μm.        SRV: lubricated with aluminum oxide as counter-body    -   The coefficients of friction are all in the range between 0.18        and 0.20 except for B1 (0.21).    -   The layer wear is very high in the case of B1, medium in the        case of C1 and E1 and low with A1. No layer wear can be observed        in the case of layers D1, C2 and D2 which are based on chromium        oxide and aluminum chromium oxide.    -   The test generates little wear of the counter-body for most        layers and no wear for A1, D1 and D2.

Finally, the test was also carried out at higher temperatures. Adifferent, somewhat more temperature-stable oil2 was used for thispurpose. The use of this oil did not change anything in principle fromthe findings of the previous SRV test at room temperature. The oil2provided slightly different coefficients of friction, but did not giveany other qualitative results for the wear behavior of coating andcounter-body.

This does not mean that this is also the case for all other oils. Theuse of another additive alone can lead to drastic changes in thecoefficient of friction. However, this was not the content of theseinvestigations and therefore will not be discussed in more detail. Theoil2 used for the SRV tests at higher temperatures was stable for testsup to temperatures of 160° C.

Table 4 indicates the tested layers tested at room temperature (RT),100° C. and 160° C. The table shows only a small selection of the testedlayers and concentrates mainly on the layer materials which are thesubject of the invention.

Again, as already described above, the time course of the coefficient offriction, the layer wear and the wear of the counter-body were testedfor these layers and the counter-bodies. In the following, thisprocedure shall again be demonstrated using an example, this timewithout the light microscopic images, and only the surface profiles willbe shown. The determination of the coefficients of friction and thequalitative assessment of the layer wear and counter-body wear werecarried out analogously to the evaluations already described above.

FIGS. 15a to c show the time courses of the coefficient of friction forthe coating systems A1, D1 and D2 with the aluminum oxide ascounter-body and for the lubricated (oil2) SRV test. A comparison of thedifferent coating materials at room temperature reveals cleardifferences between A1 and D1 as well as D2. These decrease with highertemperatures, but do not disappear. For all temperatures, layers D1 andD2 have the lowest coefficients of friction.

FIG. 16 compares the layer wear and the counter-body wear for layers A1and D2. While A1 has low layer wear of value 2, D2 shows no layer wear.In addition, there is no wear of the aluminum oxide counter-body forboth layers.

In summary, the following can be said for the SRV tests at highertemperatures:

SRV: lubricated with 100Cr6 as counter-body at higher temperatures

-   -   The coefficients of friction for all investigated layers show        values around 0.2. At RT, the layers D1 and D2 have a value of        about 0.18 and 0.16, respectively, which is significantly lower        than that of A1 (0.20). At higher temperatures these differences        disappear increasingly, although the layers on the basis of        chromium oxide and aluminum chromium oxide still have the lowest        coefficients of friction.    -   A1 shows low layer wear for all temperature ranges, whereas the        oxide layers D1 and D2 show no layer wear. On the other hand,        many non-oxidic layers are subject to wear.

With the exception of B1 and C2, there is only little wear on thecounter-body. Here, the surface roughness of the layer, and probablyalso the run-in behavior of the tribo-system, play an important role.After a certain period of time, the counter-body wear stabilizes overthe size of the contact surface.

SRV: lubricated with aluminum oxide as counter-body at highertemperatures

-   -   The coefficients of friction at RT are slightly lower and show a        small increase for the higher temperatures. However, in all        cases they are around 0.2, wherein the layers D1 and D2 on the        basis of chromium oxide and aluminum chromium oxide are lower        than in the case of A1.    -   Low layer wear occurs in the case of A1 at all temperatures,        greater wear occurs in the case of B1. The layers on the basis        of chromium oxide and aluminum chromium oxide have no wear.    -   No wear of the counter-body can be measured for all layers.

A tribological system 1 or 2 having bodies 105, 107 and counter-bodies103, 111, which together form a tribological contact, has beendescribed, the surface 108 of the body 105, 107 being coated with afirst coating 12 at least in the region of contact and the surface 112of the counter-body 103, 111 being coated with a second coating 11 atleast in the region of contact, characterized in that at least one ofthe first and second coatings 12, 11 is a layer on the basis of chromiumoxide or aluminum chromium oxide.

In the tribological system 1 or 2, the other layer 11, 12 can comprise alayer on the basis of Mo, MoN, MoCuN, DLC or ta-C.

In the tribological system 1 or 2, both the first and the second coating12, 11 can comprise layers on the basis of chromium oxide or aluminumchromium oxide.

At least one of the coatings 12, 11 and preferably both coatings 12, 11can have a chemical composition of (Al1-xCr-y)2O3, where 0.1≤x≤1 andy≤0.5.

At least one of these coatings 12, 11 and preferably both coatings 12,11 may have a hardness of 18 GPa or greater.

The tribological system can be part of an internal combustion engine100, wherein the internal combustion engine is preferably designed forthe combustion of gasoline, diesel or gas.

For example, the body can be a piston ring 103 and the counter-body canbe e.g. a cylinder running surface 108. Preferably, other surfaces ofthe internal combustion engine 100 are coated with layers on the basisof chromium oxide or aluminum chromium oxide, especially cylinder ringgrooves 104, the piston skirt (piston connector 111), piston connectorslide way 113 and/or piston crown 109 are preferred.

TABLE 1a Layers produced by thermal spraying methods and intendedprimarily for coating the cylinder running surface. Surface Designationof Chemical composition roughness the coating Powder material of thecoating Ra [μm] A1 Fe-based e(bal.)-C(1.2 wt. %)—Mn(1.4 wt. %)—Cr(1.4wt. %) 0.5 B1 Titanium oxide TiO2 0.25 C1 Ni-based Ni(bal.)—CrWMoCuCB0.33 D1 Chromium oxide Cr2O3 0.15 E1 Aluminum (Al2O3(62 wt. %)—Cr2O3(38wt. %) 0.45 chromium oxide

TABLE 1b Layers produced by cathodic spark evaporation and intendedprimarily for coating the piston rings. Surface Designation of Chemicalcomposition roughness the coating Coating of the coating Ra [μm] C2Aluminum Al—Cr—O Unpolished chromium oxide Example: 0.4 (Al0.7Cr0.3)2O3D2 Aluminum Al—Cr—O Polished chromium oxide Example: 0.15(Al0.7Cr0.3)2O3

TABLE 2 Description of the results obtained with the 100Cr6 and aluminacounter-bodies in the SRV test under dry conditions. Desig- Coefficientnation of friction Layer wear Counter-body wear of the Aluminum AluminumAluminum coating 100Cr6 oxide 100Cr6 oxide 100Cr6 oxide A1 0.86 0.73 2 44 1 B1 0.82 1.04 1(—) 5 4 1 C1 0.79 0.74 4 4 3-4 1 D1 0.66 0.66 1 1 4 1E1 0.78 0.78 1 5 4 1 C2 0.76 0.56 1(—) 1 4 1 D2 0.76 0.56 1-2 1 3 1

TABLE 3 Description of the results obtained by the SRV test underlubricated conditions (lubricant oil1) with the 100Cr6 and aluminacounter-bodies. Desig- Coefficient nation of friction Layer wearCounter-body wear of the Aluminum Aluminum Aluminum coating 100Cr6 oxide100Cr6 oxide 100Cr6 oxide A1 0.20 0.19 2 2 1-2 1 B1 0.16 0.21 2 5 2 1-2C1 0.20 0.18 2 4 2 1-2 D1 0.16 0.18 1 1 2 1 E1 0.18 0.18 1 3 3 2 C2 0.170.18 1 1 4 2 D2 0.18 0.18 1 1 2 1

TABLE 4 Description of the results obtained by the SRV test underlubricated conditions (lubricant oil2) with the 100Cr6 and aluminacounter-bodies as a function of temperature. Desig- Coefficient nationof friction Layer wear Counter-body wear of the Aluminum AluminumAluminum coating 100Cr6 oxide 100Cr6 oxide 100Cr6 oxide RT A1 0.21 0.202 2 2 1 B1 0.20 0.19 4 3-4 3 2 D1 0.18 0.18 2 9 2 1 C2 0.18 0.20 1 1-23-4 1 D2 0.16 0.16 1 1 1-2 1 100° C. A1 0.21 0.21 2 2 2 1 B1 0.22 0.21 33 2 1 D1 0.19 0.20 1 1 1-2 1 C2 0.19 0.19 1 1 3 1 D2 0.18 0.18 1 1 1-2 1160° C. A1 0.21 0.21 2 2 2 1 B1 0.21 0.21 3 3 2 1 D1 0.20 0.20 1 1 2 1C2 0.20 0.20 1 1 3 1 D2 0.19 0.19 1 1 1-2 1

The invention claimed is:
 1. A tribological system comprising a pistonand a cylinder, which each form a component of an internal combustionengine, wherein a surface of the piston comprises a first material area,wherein a surface of the cylinder comprises a second material area,wherein at least a portion of each of the first and second materialareas are configured to come into contact with each other duringoperation and form a tribological contact, wherein the first materialarea is formed as a layer on the basis of a select one of chromium oxideand aluminum chromium oxide and wherein the second material area is amixed alumina-chromium oxide layer having a composition of 10 wt % tobelow 100 wt % Cr₂O₃ and correspondingly more than 0 wt % and up to 90wt % Al₂O₃.
 2. The tribological system of claim 1, wherein the firstmaterial area is formed on at least a portion of a select one or more ofa piston ring, a piston connector, a piston ring groove area, and apiston crown of the piston.
 3. The tribological system of claim 1,wherein the second material area is formed on at least a portion of acylinder liner of the cylinder.
 4. The tribological system of claim 1,wherein the first and second material areas are each arranged on asurface of the piston and the cylinder forming a running surface,wherein the running surface comprises a select one or more of a pistonshaft slide way, a ring running surface, and a cylinder slide way. 5.The tribological system of claim 1, wherein the first material area is alayer on the basis of aluminum chromium oxide having a chemicalcomposition of (Al1-xCr-y)₂O₃, wherein: 0.1≤x≤1 and y≤0.5.
 6. Thetribological system of claim 5, wherein 0.5≤x≤1.
 7. The tribologicalsystem of claim 5, wherein x=0.7 and y=0.3.
 8. The tribological systemof claim 1, wherein the first material area has a surface roughness (Ra)of 0.1 μm to 0.5 μm and the second material area has a surface roughness(Ra) of 0.1 μm to 0.5 μm.
 9. The tribological system of claim 8, whereinthe first material area has a surface roughness (Ra) of 0.15 μm to 0.4μm.
 10. The tribological system of claim 8, wherein the second materialarea has a surface roughness (Ra) of 0.15 μm to 0.45 μm.
 11. Thetribological system of claim 1, wherein a select one or both the firstand second material areas is formed by a thermal spraying method. 12.The tribological system of claim 11, wherein a select one or both of thefirst and second material areas has a layer thickness between 150 μm and800 μm.
 13. The tribological system of claim 1, wherein a select one orboth of the first and second material areas is formed by a physicalvapor deposition (PVD) method.
 14. The tribological system of claim 13,wherein the physical vapor deposition (PVD) method comprises cathodicspark evaporation.
 15. The tribological system of claim 13, wherein aselect one or both of the first and second material areas has a layerthickness of 10 μm and 30 μm.
 16. The tribological system of claim 1,wherein the second material area is formed on the basis of a select oneor more of Mo, MoN, MoCuN, DLC, and ta-C.
 17. The tribological system ofclaim 1, wherein at least one of the first and second material areas hasa hardness of at least about 18 GPa.
 18. An internal combustion enginecomprising: a piston comprising a surface having a first material layer;and a cylinder comprising a surface having a second material layer;wherein at least a portion of the first material layer and the secondmaterial layer are in contact with each other during operation of theinternal combustion engine to form a tribological contact, wherein thefirst material layer is formed on the basis of a select one of chromiumoxide and aluminum chromium oxide and wherein the second material layeris a mixed alumina-chromium oxide layer having a composition of 10 wt %to below 100 wt % Cr₂O₃ and correspondingly more than 0 wt % and up to90 wt % Al₂O₃.
 19. A tribological system that comprises a first body anda second body, which each form a component of an internal combustionengine, wherein the first body comprises a first surface having a firstmaterial area, wherein the second body comprises a second surface havinga second material area, wherein at least a portion of the first andsecond material areas are in contact with each other during operation ofthe internal combustion engine and form a tribological contact, whereinthe first material area is formed on the basis of a select one ofchromium oxide and aluminum chromium oxide and wherein the secondmaterial area is a mixed alumina-chromium oxide layer having acomposition of 10 wt % to below 100 wt % Cr₂O₃ and correspondingly morethan 0 wt % and up to 90 wt % Al₂O₃.